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 Post subject: First Arc Flash Study, Category #0 Questions
PostPosted: Thu Jul 24, 2008 7:05 am 
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I am an engineering intern finishing my 4th year of school in EE. My employer is having me do an arc flash study of their manufacturing facility using EasySolv software. I have completed the study on one branch of their switchgear (all 480v 3-phase) and it appears that anything downstream of a 200A fuse is category #0. Do I need to go into further detail since cat #0 is the lowest category? Can I assume this for the other 4 branches of switchgear? As far as labeling these cat #0 locations will a generic label suffice? Thanks for helping a new engineer!


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PostPosted: Thu Jul 24, 2008 8:14 am 
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You might get a few other opinions from some of the others here but IMHO it is probably OK. The only thing I could think of is if there is long run of conductor where the fault current becomes so low that the fuse does not current limit. However, the current would be so low, the arc flash energy would likely also be very low.

The switchgear main might be an issue. If this is fed from a transformer, quite often the next device upstream that would limit the arc flash is the transormer primary which does not always operate as fast (if at all) for secondary faults. This could be a problem at the main that needs evaluated.

I would suggest that you run a few extra test cases to make sure you are comfortable with all the results. Not looking far enough is a common mistake and then problems can surface later as a result.


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PostPosted: Thu Jul 24, 2008 2:43 pm 
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jeppen wrote:
Do I need to go into further detail since cat #0 is the lowest category?


Yes, dont make assumptions, as K. Engholm mentioned the lower fault currents may cause an increase in Ei, but in a 480V system they can and will still be high enough to be self sustaining. If you are looking for a cutoff point use the IEEE 1584 cutoff.

jeppen wrote:
Can I assume this for the other 4 branches of switchgear?


Not at all, the systems are most likely not the same (Loads, Z, motors, setings, etc..) so the Ei's will be different in most cases. Anyone can come in and make assumptions, be an engineer!

jeppen wrote:
As far as labeling these cat #0 locations will a generic label suffice? Thanks for helping a new engineer!


Today, yes, by the time you finish this project and install the labels, no. The 2009 70E, due out in Oct 2008 (I will believe that when I see it) will have minimum info required on the labels over and above the current NFPA 70 requirements. Either the PPE required or the Ei will need to be on the labels.


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PostPosted: Tue Nov 11, 2008 7:30 pm 
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Zog,
I see that the new 2009 is out. I have looked thru a copy and did not see any labeling info. Do you know what section that label detail is located in?

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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PostPosted: Wed Nov 12, 2008 7:29 am 
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wbd wrote:
Zog,
I see that the new 2009 is out. I have looked thru a copy and did not see any labeling info. Do you know what section that label detail is located in?


Look at 130.3 (C)


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PostPosted: Wed Nov 25, 2009 1:59 pm 
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Hazard Risk Category Zero

Due to the time-current profile of MCCBs a Hazard Risk Category zero may only be possible over a small current range of the Instantaneous region of the curve. A reduction in arcing current thru the MCCB could be into the LTPU region and result in a much longer clearing time and hence 'Incident Energy'. An increase in arcing current would result in a higher Hazard Risk Category than zero. Can anyone give a typical value of current for a self-sustaining arc at 415V (line-line), 50Hz ?


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PostPosted: Wed Nov 25, 2009 6:15 pm 
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jeppen wrote:
Do I need to go into further detail since cat #0 is the lowest category? Can I assume this for the other 4 branches of switchgear? As far as labeling these cat #0 locations will a generic label suffice? Thanks for helping a new engineer!


IMO, yes, you should go further. I have seen several reasons.
1. 480 to 400v secondary will produce a higher Ie..
2. NEC tap rule violations can be found at a lot of industry (e.g. 800a main, with taps from the main cable of 8awg to the line side of a 60a fused switch.)
Most transformers that reduce the voltage will have a higher Ie on the secondary.

If your looking for total safety IMO, look at downstream device duty violations. Look for improper fusing or circuit breaker coordination. I have found several fuses in plants with old fuses (low IR on high SCC faults). So looking downstream is important.

Question #2 assume other switchgear as cat 0. This could be a dangerous assumption. IMO, model all devices to the most downstream device. You should have a good one-line to the downstream device anyways.


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